It is known to prepare Schiff's bases which contain amino groups by heating cyclic lactams, or their ring-opened polymers, such as .delta.-valerolactam, .epsilon.-caprolactam, n-capryllactam, nylon-4, nylon-6 or nylon-8, at temperatures below 260.degree. C. in the presence of an alkaline earth metal hydroxide, followed by heating the reaction mixture to at least 300.degree. C. before removal of the Schiff's bases by distillation.
According to the processes described in DE-OS (German Published Specification) No. 1,420,044 and the Japanese Application Nos. 150,832, 42,088/66, 41,766/66 and in DE-OS (German Published Specification) No. 2,303,348, cyclic lactams, or polyamides which are obtainable from the cyclic lactams by polycondensation, are heated with bases, principally calcium oxide, without any solvent to temperatures in the range from 250.degree. to 500.degree. C., and the volatile fractions are removed by distillation.
The carrying out of this reaction is associated with considerable difficulties, even on the laboratory scale, because the reaction mixture is transformed during the reaction into a solid, hard mas, from which the volatile fractions have to be removed by distillation. This means that stirred vessels and similar reactors are unsuitable for carrying out this reaction. Only elaborate apparatus, such as kneaders, is suitable for this purpose.
A modification of this preparation method is described in French Pat. No. 1,475,526. According to this patent specification, the reaction between the cyclic lactams and the alkaline earth oxides is carried out in a fluidized bed. This process represents a very elaborate technique which is characterized by high investment costs and difficulty of operation.